Aromatic carboxylic acid production involves catalytic oxidation of alkyl aromatics. The catalysts used in these processes are mainly transition metals or transition metal compounds. From process optimization point of view, it is crucial to have valuable chemicals and catalysts recovered and recycled without affecting the quality of the final product.
Terephthalic acid is produced by reacting p-xylene with molecular oxygen in the presence of a catalyst with acetic acid as solvent. The reaction results in formation of water and other by-products in dissolved form. Acetic acid gets diluted in water, formed as a side product and therefore, needs to be purified before it is recycled into the p-xylene oxidation section as unpurified/diluted acetic acid hampers the rate of p-xylene oxidation.
The solvent recovery area in the terephthalic acid plant consists of a plurality of high temperature flashing and evaporation units. The main function of this plurality of units is to recover the acetic acid from the mother liquor of p-xylene oxidation. The organic impurities that are formed as co and/or side-products during p-xylene oxidation, in high concentrations, impact both the quality and utilization of the plant. The solvent recovery section of the terephthalic acid plant is a highly energy intensive unit and high impurity concentration in this network causes frequent breakdowns and necessitates washing with caustic soda. Further, during solvent recovery, the mother liquor is subjected to high temperature flashing and evaporation for recovering acetic acid. The residue containing benzoic acid with other organic acid impurities, terephthalic acid and its oxidation intermediates and the catalyst, is flaked and sold off as crude benzoic acid. This result in the loss of the precious catalyst, that can be reused.
WO2010032263 suggests a process for recovering catalyst from the waste stream after flashing and evaporation of acetic acid. Initially, the reactor effluent produced during the manufacture of terephthalic acid is diluted with water in the weight ratio 1:1 to 1:12. Subsequently, the diluted effluent is chilled to a temperature ranging from 5 to 20° C. under stirring followed by separating the aqueous phase rich in the spent oxidation catalyst from the solid phase rich in organic compounds. Finally, the aqueous phase is concentrated to recover the spent oxidation catalyst and water. The process according to WO2010032263 includes a step of chilling the effluent, which adds up to the overall process costs.
In view of the above, there exists a need to have a catalyst recovery and recycle process having low energy and utility requirements.